1. Field of the Invention
The present invention relates to a semiconductor light emitting device and a method of producing the same, more particularly relates to a semiconductor light emitting device having a plurality of semiconductor light emitting elements for emitting a plurality of types of light having different wavelengths and a method of producing the same.
2. Description of the Related Art
An optical pickup is generally built into apparatuses for reading (reproducing) information recorded on a Compact Disk (CD), a Digital Video Disk (DVD), a Mini Disk (MD), or other optical recording medium optically recording information (hereinafter also referred to as an optical disk) or for writing (recording) information on the same (hereinafter also referred to as optical disk apparatuses).
In the above optical disk apparatuses and optical pickups, different wavelengths of laser light are generally used in different kinds of optical disks (optical disk systems). Laser light having a wavelength of 780 nm is used for playing a CD, while laser light having a wavelength of 650 nm is used for playing a DVD.
Under these conditions, where different wavelengths of laser light are used depending on the kinds of the optical disks, a compatible optical pickup which enables, for example, a CD to be played in a DVD optical disk apparatus has been desired.
FIG. 1 is a view of the configuration of a compatible optical pickup provided with a CD laser diode LD1 (emission wavelength of 780 nm) and a DVD laser diode LD2 (emission wavelength of 650 nm) so as to enable playback of a CD and DVD as a first example of the related art.
An optical pickup 100 includes a separately configured, that is, discretely composed CD optical system comprised of, for example, a first laser diode LD1 for emitting laser light having a wavelength of for example the 780 nm band, a grating G, a first beam splitter BS1, a first mirror M1, a first object lens OL1, a first multiple lens ML1, and a first photodiode PD1, arranged at predetermined positions.
Furthermore, the above optical pickup 100 includes a DVD optical system comprising, for example, a second laser diode LD2 for emitting laser light having a wavelength of for example the 650 nm band, a second beam splitter BS2, a collimeter C, a second mirror M2, a second object lens OL2, a second multiple lens ML2, and a second photodiode PD2 arranged at predetermined positions.
In the CD optical system of the optical pickup 100 of the above configuration, the first laser light L1 from the first laser diode LD1 passes through the grating G, is partially reflected by the first beam splitter BS1, is bent in course by the first mirror M1, and then is converged on an optical disk D by the first object lens OL1.
The reflected light from the optical disk D passes through the first multiple lens ML1 via the first object lens OL1, the first mirror M1, and the first beam splitter BS1 and strikes the first photodiode PD1. The information recorded on the CD recording surface of the optical disk D is read by the changes of this reflected light.
In the DVD optical system of the optical pickup 100 of the above configuration as well, in the same way as the above, the laser light L2 from the second laser diode LD2 is partially reflected by the second beam splitter BS2, passes through the collimeter C, is bent in its course by the second mirror M2, and then is converged on the optical disk D by the second object lens OL2.
The reflected light from the optical disk D passes through the second multiple lens ML2 via the second object lens OL2, second mirror M2, collimeter C, and second beam splitter BS2 and strikes the second photodiode PD2. The information recorded on the DVD recording surface of the optical disk D is read by the changes of this reflected light.
The above optical pickup 100 makes it possible to play back both CD and DVD by providing a CD laser diode, a DVD laser diode, and respective optical systems.
FIG. 2 is a view of the configuration of a compatible optical pickup provided with a CD laser diode LD1 (emission wavelength of 780 nm) as in the above and a DVD laser diode LD2 (emission wavelength of 650 nm) so as enable playback of a CD and DVD as a second example of the related art.
An optical pickup 101 contains a separately configured, that is, discretely composed CD optical system comprising, for example, a first laser diode LD1 for emitting laser light having a wavelength of for example the 780 nm band, a grating G, a first beam splitter BS1, a dichroic beam splitter DBS, a collimeter C, a mirror M, a CD aperture R, an object lens OL, a first multiple lens ML1, and a first photodiode PD1, arranged at predetermined positions.
Furthermore, the above optical pickup 101 contains a DVD optical system comprising, for example, a second laser diode LD2 for emitting laser light having a wavelength of for example the 650 nm band, a second beam splitter BS2, the dichroic beam splitter DBS, the collimeter C, the mirror M, the object lens OL, a second multiple lens ML2, and a second photodiode PD2 arranged at predetermined positions.
In the above optical systems, some of the optical members are used in common, for example, the dichroic beam splitter DBS, the collimeter C, the mirror M, and the object lens OL are commonly used by the two optical systems. Since the dichroic beam splitter DBS and the optical disk D share the same optical axis, a CD aperture R is provided on the optical axis of the DVD optical system as well.
In the CD optical system of the optical pickup 101 of the above configuration, the first laser light L1 from the first laser diode LD1 passes through the grating G, is partially reflected by the first beam splitter BS1, passes through or is reflected at the dichroic beam splitter DBS, the collimeter C, and the mirror M, and then is converged on the optical disk D by the object lens OL via the CD aperture R.
The reflected light from the optical disk D passes through the first multiple lens ML1 via the object lens OL, the CD aperture R, the mirror M, the collimeter C, the dichroic beam splitter DBS, and the first beam splitter BS1 and strikes the first photodiode PD1. The information recorded on the CD recording surface of the optical disk D is read by the change of this reflected light.
In the DVD optical system of the optical pickup 101 of the above configuration as well, in the same way as the above, the laser light L2 from the second laser diode LD2 is partially reflected by the second beam splitter BS2, passes through or is reflected at the dichroic beam splitter DBS, the collimeter C, and the mirror M, and then is converged on the optical disk D by the object lens OL via the CD aperture R.
The reflected light from the optical disk D passes through the second multiple lens ML2 via the object lens OL, the CD aperture R, the mirror M, the collimeter C, the dichroic beam splitter DBS, and the second beam splitter BS2 and strikes the second photodiode PD2. The information recorded on the DVD recording surface of the optical disk D is read by the changes in this reflected light.
According to the above optical pickup 101, in the same way as in the optical pickup 100 shown in FIG. 1, it is made possible to play back both CD and DVD by providing a CD laser diode and a DVD laser diode and respective optical systems.
Summarizing the problems to be solved by the present invention, the above optical pickups have a large number of parts and the optical systems are complicated in configuration, so it is not easy to assemble the pickups, it is difficult to make the optical device compact, and, furthermore, the cost inevitably becomes higher.
In the above optical pickups of the related art, one of the reasons why the number of parts was large and the optical system became complex in configuration was that the CD laser diode and the DVD laser diode were separately provided.
An example of a laser diode used in the above optical pick up system is shown in FIG. 3 by a cross-sectional view.
For example, an n-type GaAs buffer layer 31, an n-type AlGaAs clad layer 32, an active layer 33, a p-type AlGaAs clad layer 34, and a p-type GaAs cap layer 35 are stacked on an n-type GaAs substrate 30. A stripe forming a current narrowing structure is formed as an insulated region 41 from the surface of the p-type GaAs cap layer 35 to the middle of the p-type AlGaAs clad layer 34.
Also, a p-type electrode 42 is formed connected to the p-type GaAs cap layer 35 and an n-type electrode 43 is formed connected to the n-type GaAs substrate 30.
In a laser diode of the above configuration, one laser structure is formed by stacking, for example, an AlGaInP-based material on a GaAs substrate, or one laser structure is formed by stacking an InGaAsP-based material on an InP substrate. That is, a laser structure is formed by one kind of material on one kind of substrate, and light of a substantially constant single type of wavelength is emitted.
Also, as shown in FIG. 4, there has been developed a method of forming a first laser diode LD1 and a second laser diode LD2 on the same substrate in accordance with usage.
For example, an n-type GaAs buffer layer 31, an n-type AlGaAs clad layer 32, an active layer 33, a p-type AlGaAs clad layer 34, and a p-type GaAs cap layer 35 are stacked on an n-type GaAs substrate 30. A first laser diode LD1 is formed by forming a stripe forming a current narrowing structure by an insulated region 41 from the surface of the p-type GaAs cap layer 35 to the middle of the p-type AlGaAs clad layer 34.
On the other hand, the second laser diode LD2 has substantially the same configuration. The composition of an active layer 33xe2x80x2 is basically the same as that of the active layer 33 of the first laser diode LD1, so the wavelengths of laser light emitted are almost the same (if not, the difference is very small).
Furthermore, a p-type electrode 42 is formed connected to the p-type GaAs cap layer 35, and an n-type electrode 43 is formed connected to the n-type GaAs substrate 30.
However, in the first laser diode LD1 and the second laser diode LD2 of the above configuration, the wavelengths of the light emitted from the two laser diodes are the same or, even if not the same, the difference is very small. Therefore, they cannot be used, for example, for a CD laser diode and a DVD laser diode.
An object of the present invention is to provide a semiconductor light emitting device having a plurality of semiconductor light emitting elements of different emission wavelengths enabling an optical pickup of a CD, DVD, or other different wavelength optical disk system to be reduced in the number of parts, simplified in the configuration of the optical system, easily assembled, and made more compact and lower in cost.
To attain the above object, according to a first aspect of the present invention, there is provided a semiconductor light emitting device having a plurality of semiconductor light emitting elements on a substrate, comprising a substrate and at least two stacks each formed on the substrate and comprised of an epitaxially grown layer comprising at least a first conductivity type clad layer, an active layer, and a second conductivity type clad layer stacked together; the stacks being spatially separated from each other; the composition of at least the active layers being different between the stacks; and a plurality of types of light of different wavelengths from each other being emitted from the active layers to a direction parallel to a surface of the substrate.
Since the above semiconductor light emitting device of the present invention provides on a substrate at least two stacks comprised of epitaxial growth layers each comprising at least a first conductivity type clad layer, an active layer, and a second conductivity type clad layer and the compositions of the active layers differ between the stacks, it is possible to construct a monolithic semiconductor light emitting device capable of emitting a plurality of types of light having different wavelengths from the active layers.
The semiconductor light emitting device of the present invention preferably emits a plurality of types of laser light having respectively different wavelengths from the active layers. By this, it is possible to provide a laser diode enabling an optical pickup of a CD, DVD, or other different wavelength optical disk system to be reduced in number of parts, simplified in configuration, easily assembled, and made compact and lower in cost.
The semiconductor light emitting device preferably is one in which the ratios of composition of the active layers are different between the stacks. Alternatively, it is one in which the compositions of the active layers include different elements between the stacks. Alternatively, it is one in which the compositions of the first conductivity type clad layers, the active layers, and the second conductivity type clad layers are different between the stacks. It therefore becomes possible to make the wavelengths of the light emitted from the active layers different.
The semiconductor light emitting device preferably is one wherein types of light of different polarization directions are emitted from the active layers. Since provision is made on a substrate of at least two stacks each comprising a first conductivity type clad layer, an active layer, and a second conductive clad layer, it is possible to configure semiconductor light emitting elements to emit light of different polarization directions on the same substrate.
The semiconductor light emitting device is preferably one comprising, as the stacks, a first stack and a second stack, the first stack and second stack being formed above the substrate. More preferably, the substrate is a first conductivity type; and the first stack and second stack are formed stacked above the substrate from a side of the first conductivity type clad layers and electrically connected using the substrate as a common electrode. It therefore becomes possible to provide a plurality of stacks directly above the substrate.
Alternatively, the semiconductor light emitting device is preferably one comprising, as the stacks, a first stack and a second stack; the second stack being formed above the first stack. More preferably, the substrate is a first conductivity type; and the second stack is formed stacked above the first stack in a region made the first conductivity type from a side of the first conductivity type clad layer and is formed electrically connected to the substrate via the first stack in the region made the first conductivity type. Alternatively, more preferably, the substrate is a first conductivity type; and the second stack is formed above the first stack via a first conductivity type layer formed above a second conductivity type layer of the first stack. It therefore becomes possible to provide a further stack above a stack formed on the substrate.
The semiconductor light emitting device preferably is one wherein each of the stacks has a current narrowing structure. More preferably, a region doped with an impurity is formed in the stack to form the current narrowing structure. Alternatively, more preferably, the stack is processed to a ridge shape to form the current narrowing structure. It therefore is possible to improve the efficiency of current injection for more effective operation and to reduce the power consumption.
Further, to attain the above object, according to a second aspect of the present invention, there is provided a method of producing a semiconductor light emitting device comprising on a substrate a first semiconductor light emitting element and a second semiconductor light emitting element for emitting light of different wavelengths from each other, including the steps of forming on the substrate by an epitaxial growth method a first stack comprised of at least a first clad layer of a first conductivity type, a first active layer, and a second clad layer of a second conductivity type; removing the parts of the first stack other than the part at the first semiconductor light emitting element formation region; forming on the substrate by an epitaxial growing method a second stack comprised of at least a third clad layer of the first conductivity type, a second active layer, and a fourth clad layer of the second conductivity type; and removing the parts of the second stack other than the part at the second semiconductor light emitting element formation region; at least the first active layer and the second active layer being formed by different compositions from each other.
In the method of producing the semiconductor light emitting device of the present invention, that is, an epitaxial growth method is used to form on a substrate a first stack comprising at least a first clad layer of a first conductivity type, a first active layer, and a second clad layer of a second conductivity type. Next, the parts of the first stack other than the part of the first stack at a first semiconductor light emitting element formation region are removed. Next, an epitaxial growth method is used to form on the substrate a second stack comprising at least a third clad layer of the first conductivity type, second active layer, and a fourth clad layer of the second conductivity type. Here, the first active layer and second active layer are formed by different compositions from each other. Next, the parts of the second stack other than the part of the second stack at a second semiconductor light emitting element formation region are removed.
According to the above method of producing the semiconductor light emitting device of the present invention, it is possible to directly form above the substrate a first stack comprising a first clad layer of a first conductivity type, a first active layer, and a second clad layer of a second conductivity type and a second stack comprising a third clad layer of the first conductivity type, a second active layer, and a fourth clad layer of the second conductivity type.
Since the compositions of the two active layers are made different between the stacks, it is possible to form a monolithic semiconductor light emitting device capable of emitting light of different wavelengths from the active layers. Therefore, the invention is suitable for the optical pickup of CD, DVD, or other different wavelength optical disk systems and also enables the number of parts to be reduced, the configuration of the optical system to be simplified, assembly to be made easy, and greater compactness and lower cost to be achieved.
Also, to attain the above object, according to a third aspect of the present invention, there is provided a method of producing a semiconductor light emitting device having on a substrate a first semiconductor light emitting element and a second semiconductor light emitting element for emitting light of different wavelengths from each other, including the steps of forming on a substrate by an epitaxial growth method a first stack comprising at least a first clad layer of a first conductivity type, a first active layer, and a second clad layer of a second conductivity type; forming on the first stack by an epitaxial growth method a second stack comprising at least a third clad layer of the first conductivity type, a second active layer, and a fourth clad layer of the second conductivity type; and removing the parts of the first stack and second stack other than the parts of the first stack and second stack at a second semiconductor light emitting element formation region and the part of the first stack at a first semiconductor light emitting element formation region; at least the first active layer and second active layer being formed by different compositions from each other.
In the above method of producing a semiconductor light emitting device of the present invention, an epitaxial growth method is used to form on a substrate a first stack comprising at least a first clad layer of a first conductivity type, first active layer, and a second clad layer of a second conductivity type. Next, an epitaxial growth method is used to form on the first stack a second stack comprising at least a third clad layer of the first conductivity type, a second active layer, and a fourth clad layer of the second conductivity type. Here, compositions of at least the first active layer and the second active layer are made different. Then, the parts of the first stack and the second stack other than the parts of the second stack and first stack at a second semiconductor light emitting element formation region and the part of the first stack at a first semiconductor light emitting element formation region are removed.
According to the above method of producing a semiconductor light emitting device of the present invention, it is possible to provide on a first stack comprising a first clad later of a first conductivity type, first active layer, and second clad layer of a second conductivity type formed on a substrate a second stack comprising a third clad layer of the first conductivity type, a second active layer, and a fourth clad layer of the second conductivity type. In such a case, since the second stack can be formed on a flat surface (on upper surface of the first stack), the epitaxial growth becomes easy.
Since the compositions of the two active layers are made different between the stacks, it is possible to form a monolithic semiconductor light emitting device capable of emitting light of different wavelengths from the active layers. Therefore, the invention is suitable for an optical pickup of a CD, DVD, or other different wavelength optical disk system and provides a laser diode which enables the number of parts to be reduced, the configuration of the optical system to be simplified, assembly to be made easy, and greater compactness and lower cost to be achieved.
The method of producing the semiconductor light emitting device of the present invention preferably is one further including the step of making the first stack at the second semiconductor light emitting element formation region a first conductivity type prior to the step of forming the second stack; and in the step of forming the second stack, forming the second stack above the first stack made the first conductivity type from a side of the third clad layer of the first conductivity type of the second stack.
It is therefore possible to form the second stack so as to be connected to the substrate via the first stack made the first conductivity type.
The above methods of producing the semiconductor light emitting device of the present invention preferably are ones wherein the first active layer and second active layer are formed to have mutually different ratios of composition. Alternately, they are ones wherein the first active layer and second active layer are formed by mutually different composition elements. Alternately, they are ones wherein the compositions of the first clad layer of the first conductivity type, first active layer, and second clad layer of the second conductivity type and the compositions of the third clad layer of the first conductivity type, second active layer, and fourth clad layer of the second conductivity type are made different from each other. As a result, it is possible to make the wavelengths of light emitted from the active layers mutually different.